Where differential Satellite Positioning System (DSATPS) signals are combined with SATPS signals to enhance the accuracy of the present location of a mobile user, such as a marine or airborne vessel or land vehicle, one concern is how to monitor the quality or integrity of the signals used in the location computations. Monitoring of signal quality or integrity has thus far used calculations based only on SATPS pseudorange signals. This approach has built-in limitations arising from the fact that the same data are being used to compute SATPS-determined location and to evaluate the quality or integrity of the underlying SATPS signals.
Although measurements and use of pseudoranges are fundamental to S ATPS-assisted determination of location and/or time coordinates, only a few patents disclose procedures that work directly with the pseudorange or pseudorange rate values. In U.S. Pat. No. 4,578,678, Hurd discloses a GPS receiver that receives a plurality of pseudorange signals, compares these signals with replicas of the expected pseudorange signals, using a correlation technique, and determines the associated time delay, frequency and other variables of interest for these signals to determine receiver location, velocity, clock offset and clock rate.
Several references discuss monitoring one or more parameters associated with a location determination (LD) system, such as GPS or Loran-C, and use of this information in subsequent decisions. Gray et al, in U.S. Pat. No. 4,651,157, disclose use of a plurality of Loran-C or satellites for determining the location of a land-based, marine or airborne vehicle by a receiver/sensor/transmitter (RST) carried on the vehicle. This RST receives the LD signals from the Loran-C or satellite transmitters and retransmits these signals and the values of one or more monitored parameters to a central station that processes this information and determines the RST's present location.
In U.S. Pat. No. 4,791,572, Green et al disclose a system for providing differential corrections to locations determined by a Loran-C LD system-that is analogous to a well known system for providing differential corrections for GPS. Another Loran-C differential positioning system is disclosed by Duffet-Smith in U.S. Pat. No. 5,045,861.
Use of a plurality of GPS receivers and antennas to accurately determine the location of a seismic survey vessel is disclosed by Counselman in U.S. Pat. No. 4,809,005. L1 and L2 band carrier waves are received and used (1) to correct for ionospheric time delay and (2) to determine a biasfree pseudorange from each GPS satellite to a given receiver, using signal time averaging. A plurality of receivers, spaced apart from each other on the vessel, is used to compensate for signal blocking by the vessel, to sense and compensate for false signals, and to receive GPS signals by at least one receiver at all times. Another patent issued to Counselman, U.S. Pat. No. 4,894,662, also identifies bias and ionospheric time delay in the pseudorange signals, using C/A signals.
Olsen et al disclose a GPS-based geophysical survey system the includes a fixed GPS reference station and a plurality of mobile survey stations that also use GPS signals, in U.S. Pat. No. 4,814,711. The reference station transmits time-varying signals indicating the desired location of each mobile station. Each mobile station receives these desired-location signals, receives GPS signals, determines the actual location of that mobile station, and periodically transmits to the reference station this actual location and the survey parameters sensed or measured by the mobile station. The reference station compares the actual location and desired location for each reporting mobile station and correlates the reported geophysical information with the corresponding location of that mobile station.
A system that measures velocity of a given object relative to a fixed surface by using Doppler shifts of radio waves received by two receiver/sensor/transmitter combinations is disclosed by Stratton et al in U.S. Pat. No. 4,893,287. Two RSTs face each other and are preferably directed toward the same location on the surface, and the system assertedly random, non-real velocity values by comparison of velocity components at a sequence of times.
Bice et al disclose an aircraft ground collision avoidance system in U.S. Pat. No. 4,924,401. Each aircraft carries an autopilot that monitors "flight states." such as aircraft airspeed. angle of attack, bank angle and velocity coordinates.
In U.S. Pat. No. 4,970,523, Braistead et al disclose a system that determines differential Doppler frequency shifts received at a vehicle and estimates the present vehicle velocity from these differential shifts.
Keegan discloses a P-code receiver/processor, in U.S. Pat. No. 4,972,431, that analyzes pseudorange and phase for encrypted GPS signals by squaring and filtering the incoming signals. Weaker Signals can be analyzed using this technique.
A system for determining the location, orientation and velocity of an airborne vehicle is disclosed in U.S. Pat. No. 4,990,922, issued to Young et al. The system uses two or more antennas, spaced apart on the vehicle to receive GPS signals for this purpose. Post processing of the GPS signal information is performed at a central station.
U.S. Pat. No. 5,017,930, issued to Stoltz et al, discloses an aircraft precision landing system that uses four or more signal receivers at fixed, known locations and a centrally located radio signal source. This source transmits an interrogation signal that is received and answered by transponders on the aircraft as the aircraft approaches a landing site. A central station receives the transponder signals and determines the aircraft's computed location along an approach path is compared with the desired path. Location errors are communicated to the aircraft by the central station so that the aircraft can make appropriate adjustments in its present approach path.
A system for monitoring and reporting on the present location of a vehicle generally traveling along a prescribed route is disclosed by Sutherland in U.S. Pat. No. 5,068,656. The vehicle determines its location, compares the present vehicle location with the desired location, and transmits exception reports to a central station if these two locations differ by more than a threshold value.
Allison, in U.S. Pat. No. 5,148,179, discloses a method for using double differences of pseudorange and carrier phase measurements. The technique uses double differences formed from signals received from four satellites by two different receivers to eliminate certain bias and atmospheric perturbation terms.
Harigae et al, in U.S. Pat. No. 5,153,599, disclose clock testing apparatus, connected to a GPS signal receiver on a moving station, that counts clock pulses issued by this receiver. The system determines a receiver clock error and models error in the velocity of the moving station relative to each GPS satellite.
A GPS receiver that uses conventional pseudorange and carrier phase measurements to provide a directional indicator, such as a compass, with improved accuracy is disclosed in U.S. Pat. No. 5,266,958, issued to Durboraw. A single antenna is moved in a closed path, and differences between predicted and actual carrier phases are used to determine location perturbations, which are then resolved into components parallel and perpendicular to a desired path heading in a given plane.
None of the references discussed above examines and uses the pseudorange rate signals derived from Doppler or carrier phase information used to compute pseudorange rate corrections for a DSATPS reference station or to monitor the integrity of the signals received by a DSATPS reference station or by an associated signal integrity monitoring station. Where a reference discusses use of velocity information, it is usually the mobile station velocity that is determined or compared. Only a few U.S. patents innovatively use pseudorange or pseudorange rate signals for any purpose.
What is needed is a system that uses data for signal integrity monitoring that are at least partly independent of the signals used for computation of the pseudorange corrections by a DSATPS reference station. Preferably, independent signal integrity monitoring at a nearby fixed station should be provided to identify or detect anomalous differential correction signals that arise at the reference station and/or anomalous signals used in the computation of the location and/or velocity of the nearby station.